Conventionally, in a backlight for large-sized liquid crystal display apparatuses, a number of cold cathode tubes are disposed immediately below a liquid crystal panel to be used with other members such as a diffusing plate and a reflecting plate. In recent years, LEDs have been used in common as light sources for backlights. LEDs are expected to serve as low-power light sources to replace fluorescent lamps, due to the recent improvement in their efficiency. When LEDs are used as light sources in a liquid crystal display apparatus, the power consumption of the liquid crystal display apparatus can be reduced by controlling the brightness of the LEDs according to the image to be displayed.
When LEDs are used as light sources for a backlight in a liquid crystal display apparatus, a large number of LEDs are to be provided instead of cold cathode tubes. Although the use of such a large number of LEDs may allow uniform brightness on the front surface of the backlight, the need for a large number of LEDs makes it difficult to reduce the cost, which is a problem. Attempts have been made to increase the output power of each LED to reduce the required number of LEDs. For example, Patent Literature 1 proposes a light emitting device that is designed to provide a uniform surface light source even with a reduced number of LEDs.
In order to obtain a uniform surface light source with a reduced number of LEDs, it is necessary to increase the illumination area that can be illuminated by each LED. To achieve this, the light emitting device of Patent Literature 1 uses a lens that radially expands the light from an LED. This widens the directionality of the light from the LED, thus enabling a wider range on the illumination target surface to be illuminated, with the optical axis being at the center. Specifically, the lens used for the light emitting device of Patent Literature 1 is circular in plan view, in which both a light entrance surface that is concave, and a light exit surface that is concave in the vicinity of the optical axis and is convex on the circumference thereof, are rotationally symmetric with respect to the optical axis.
Meanwhile, Patent Literature 2 discloses a light emitting device using a lens that has a light exit surface in the middle of which a V-shaped groove extending in a direction orthogonal to the optical axis is formed. According to the lens of this light emitting device, the light from the LED is expanded while the angular distribution remains a normal distribution in the direction in which the V-shaped groove extends (in the longitudinal direction). However, in the direction orthogonal to the direction in which the V-shaped groove extends (in the width direction), the light is expanded such that the angular distribution significantly drops in the vicinity of the optical axis and steeply rises on both lateral sides.